This invention relates generally to hydrocarbon processing and, more particularly, to the processing of a resulting naphtha process stream via a dividing wall separation column to form or obtain process streams composed of hydrocarbons containing particular desired ranges of carbon atoms.
A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks in the modern petrochemical and chemical industries.
Light olefins have traditionally been produced through the processes of steam or catalytic cracking of hydrocarbons such as derived from petroleum sources. Fluidized catalytic cracking (FCC) of heavy hydrocarbon streams is commonly carried out by contacting a starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst such as composed of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by transmitting a gas or vapor through the catalyst at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction.
The cracking reaction typically deposits coke on the catalyst. Catalyst exiting the reaction zone is commonly referred to as being “spent”, i.e., partially deactivated by the deposition of coke upon the catalyst. Coke is comprised of hydrogen and carbon and can include, in trace quantities, other materials such as sulfur and metals such that may enter the process with the starting material. The presence of coke interferes with the catalytic activity of the spent catalyst. It is believed that the coke blocks acid sites on the catalyst surface where the cracking reactions take place. Spent catalyst is traditionally transferred to a stripper that removes adsorbed hydrocarbons and gases from catalyst and then to a regenerator for the purpose of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the spent catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated between the reaction zone and the regeneration zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. FCC processing is more fully described in U.S. Pat. No. 5,360,533 to Tagamolila et al., U.S. Pat. No. 5,584,985 to Lomas, U.S. Pat. No. 5,858,206 to Castillo and U.S. Pat. No. 6,843,906 B1 to Eng, the contents of each of these patents are hereby incorporated herein by reference. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
Such FCC processing typically results in the formation of a product or effluent stream containing a distribution of hydrocarbon products having a range of carbon atoms. Consequently, such processing also is typically associated with hydrocarbon recovery processing to recover specified fractions or portions of the product hydrocarbons for use as is or after subsequent or additional processing. For example, ethylene and propylene can be recovered as desired products such as in the form of polymer grade feedstocks for use in corresponding or associated poly units. More specifically, cracked vapors from an FCC unit enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, light cycle oil (LCO) and clarified oil (CO) which includes heavy cycle oil (HCO) components. In conventional FCC processing, such gas stream is commonly further processed through a gas concentration system such as to produce a dry gas stream, i.e., hydrogen, C1 and C2 hydrocarbons and typically less than 5 mol % C3+ hydrocarbons, a mixed liquefied petroleum gas (“LPG”) stream, i.e., C3 and C4 hydrocarbons, also sometimes commonly referred to as wet gas and a stabilized naphtha stream. The naphtha can then be stripped to remove the C2− materials and then debutanized to remove the LPG.
In view of an increasing need and demand for light olefins such as ethylene and propylene for various petrochemical uses such as for the production of polyethylene, polypropylene and the like as well as the desire to produce relatively less of heavier olefins such as butylenes and pentenes which are generally less desirable as gasoline blending components due to environmental considerations, it may be desired to practice cracking reaction processing of heavy hydrocarbon feedstock to increase the relative amount of light olefins in the resulting product slate.
Research efforts have led to the development of an FCC process that produces or results in greater relative yields of light olefins, i.e., ethylene and propylene. Such processing is more fully described in U.S. Pat. No. 6,538,169 B1 to Pittman et al., the contents of which are hereby fully incorporated herein by reference. As disclosed therein, a hydrocarbon feed stream can desirably be contacted with a blended catalyst comprising regenerated catalyst and coked catalyst. The catalyst has a composition including a first component and a second component. The second component comprises a zeolite with no greater than medium pore size wherein the zeolite comprises at least 1 wt. % of the catalyst composition. The contacting occurs in a riser to crack hydrocarbons in the feed stream and obtain a cracked stream containing hydrocarbon products including light olefins and coked catalyst. The cracked stream is passed out of an end of the riser such that the hydrocarbon feed stream is in contact with the blended catalyst in the riser for less than or equal to 2 seconds on average.
In addition, it has been proposed that the amounts of light olefins resulting from at least certain kinds of hydrocarbon processing can be further increased by reacting, i.e., cracking, heavier hydrocarbon products, particularly heavier olefins such as C4-C6 olefins, to light olefins. U.S. Pat. No. 5,914,433 to Marker, the entire disclosure of which is fully incorporated herein by reference, discloses a process for the production of light olefins comprising olefins having from 2 to 4 carbon atoms per molecule from an oxygenate feedstock. The process comprises passing an oxygenate feedstock to an oxygenate conversion zone containing a metal aluminophosphate catalyst to produce a light olefin stream. A propylene stream and/or mixed butylene is fractionated from said light olefin stream and cracked to enhance the yield of ethylene and propylene products.
While such FCC and olefin cracking processing have proven generally effective in the production of desired light olefins, further improvements have been and are being sought. In particular, improvements in post-FCC process stream handling have been sought such as to either or both simplify and increase efficiency and/or effectiveness of desired further downstream processing. More specifically, further improvements in the processing of the resulting effluent materials particularly in producing desired sharper splits of the hydrocarbon products than have heretofore been commonly obtainable and, more particularly, doing so in a more energy efficient manner have been sought and are desired.